A protester stands in the street after police fired tear gas to disperse a crowd during a protest for Michael Brown, who was killed by ...

A demonstrator holds a sign reading, "We Demand Justice", in front of a line of police as demonstrators continue to make their voices heard in ...

A memorial setup in the place where Michael Brown Jr. was killed in Canfield Apartments in Ferguson, MO. (AFP Photo)

Demonstrators face off against the police as they are cleared from the street while protesting the shooting death of Michael Brown. (AFP Photo)

Police fired tear gas Monday in another night of unrest in a Missouri town where a white police officer shot and killed an unarmed black teenager, just hours after President Barack Obama called for calm.

US National Guard troops rolled into Ferguson earlier in the day, but they kept a low profile as police in riot gear dispersed about 100 demonstrators around 11 pm, arresting several of them.

Several rounds of gunfire -- described by a senior police officer as "potshots in the area" -- were heard, adding to tension that has gripped this St Louis suburb since the fatal shooting of 18-year-old student Michael Brown.

His death, and the heavy-handed police response, has reignited a national debate about race and law enforcement in the United States.

A protester stands in the street after police fired tear gas to disperse a crowd during a protest for Michael Brown, who was killed by a police officer in Ferguson, Missouri. (AP Photo)

Obama, the nation's first African-American president, said he was sending Attorney General Eric Holder to Ferguson on Wednesday as Washington pursues a civil rights investigation into the case.

Obama said there was no excuse for local police to employ "excessive force" and urged the state of Missouri to make only "limited" use of the National Guard.

By late afternoon, about 200 Missouri National Guard soldiers arrived in Ferguson, with snipers seen posted on rooftops near a police command center. The troops are operating under Missouri Highway Patrol supervision.

The reinforcements allowed State Governor Jay Nixon to lift an overnight curfew, but tempers were still running high amid controversy over Brown's death.

"They're supposed to protect the American citizens, but they're fighting a war with unarmed citizens," said Ron Henry, who wore a T-shirt with the phrase "stop killing us."

Amid the trouble, Getty Images photographer Scott Olson was arrested for unknown reasons, the agency said. He was later released.

As night fell, several hundred people took part in fresh protests after police with plastic handcuffs moved crowds of demonstrators into designated zones to clear a main street in the town that had been the scene of violent protests.

Protesters were not allowed stop and gather, and were directed to keep moving.

Residents were on edge, after the previous night saw rioters loot stores and throw Molotov cocktails.

Brown was shot dead in broad daylight on a residential street last Saturday by Darren Wilson, a 28-year-old white police officer.

A law enforcement officer on a tactical vehicle watches after a device was fired to disperse a crowd during a protest for Michael Brown, who was killed by a police officer in Ferguson, Missouri. (AP Photo)

A forensic pathologist retained by the victim's family revealed that the student had been shot at least six times -- twice in the head.

Different versions of the shooting have emerged, with police sources saying there was a scuffle during which Brown tried to seize the policeman's weapon, while witnesses have alleged that Brown had his hands up and was not resisting when he was shot.

The Washington Post said traces of marijuana were found in Brown's system.

A total of three autopsies have been requested -- by local authorities, the family and the Justice Department.

Officials told news media that a Missouri grand jury could hear evidence in the case as early as Wednesday.

Obama warned of a "gulf of mistrust" between residents and police in many cities and towns across America, particularly in those where racial minorities feel excluded from opportunities for a better life.

Protesters react to the effects of tear gas which was fired at demonstrators reacting to the shooting of Michael Brown in Ferguson, Missouri. (Reuters Photo)

"To a community in Ferguson that is rightly hurting and looking for answers, let me call once again for us to seek some understanding rather than simply holler at each other," he said.

"In too many communities, too many young men of color are left behind and seen only as objects of fear," Obama said.

Former New York chief medical examiner Michael Baden, who examined Brown's body on behalf of his family, said Monday he found no evidence of an alleged struggle between Brown and the officer.

Wilson is said to have been hurt in the incident, and Baden said that he had not examined the police officer.

The absence of gunpowder on Brown's body indicated that the muzzle of the gun was probably at least a foot or two away -- or as much as 30 feet (10 meters) -- Baden added.